1. Field of the Invention
The present invention relates to a steering wheel core material which is produced from FRP (fiber reinforced plastics).
2. Description of the Prior Art
For the purpose of weight reduction and strengthening, a new type of automobile steering wheel has recently been developed. It is made up of a core reinforced with continuous filament rovings, a cushioning material covering the core, and a surface layer of leather or the like. It is now in practical use.
The core is made of continuous filaments such as glass fiber and carbon fiber impregnated with epoxy resin, polyester resin, or the like. To make the core, the resin-impregnated continuous filaments are wound in layer around a jig and the resin is cured by heating. This FRP core is light in weight and very strong. It is used to produce a steering wheel which is very little subject to the vibrations of the automobile during driving.
This type of steering wheel core has a structure as shown in FIG. 25. The steering wheel core 1 of FRP is made up of a ring 2, a boss 3 positioned at the center of the ring 2, and spokes 4 which connect the ring 2 to the boss 3. The boss 3 is provided with a metal fitting 9 to which the steering shaft is connected. The metal fitting is integrally joined to the FRP core when the continuous filaments are laminated.
The steering wheel core 1 is produced by using an apparatus shown in FIGS. 26 and 27. The apparatus 5 is made up of a main shaft 6, a lower mold 7 fixed to the shaft 6, and an upper mold 8 which mates with the lower mold 7. The upper and lower molds 7 and 8 have grooves 7a and 8a, respectively, formed on their circumference. When the upper and lower molds 7 and 8 are closed, these grooves 7a and 8a form a groove having a semicircular cross-section in which the ring 2 is made. The core 1 is produced in the following manner. At first, a boss metal fitting 9 is attached to the shaft 6, and the upper mold 8 is mated to the lower mold 7. Resin-impregnated continuous filaments are wound around the groove by turning the shaft 6 so that the ring 2 is formed, and they are bent at the notches 8b formed on the circumference of the upper mold 8 and led to the boss metal fitting 9 and wound around its body 9a so that the boss 3 and spokes 4 are formed. After winding, the assembly is heated to cure the resin. Thus there is obtained the steering wheel core 1 of continuous filament roving FRP.
A disadvantage of the steering wheel core 1 produced as mentioned above is that it has such a light weight and a small inertia moment that it is poor in performance to prevent the steering wheel from vibrating in the circumferential direction (flutter characteristics).
One way to improve the flutter characteristics is to increase the weight of the ring 2 by attaching weights to the ring 2 or by increasing the amount of continuous filaments wound in the ring 2. However, attaching weights leads to low productivity and high production cost because it takes more time and labor to place weights at accurate positions on the apparatus 5 for the well-balanced steering wheel. On the other hand, increasing the amount of continuous filaments results in an extremely thick ring 2 because they have a low specific gravity. A steering wheel with a thick ring is awkward to handle. Unfortunately, improving the flutter characteristics by increasing the weight of the ring aggravates the shake characteristics because it increases the total weight of the steering wheel.